Precomposing/ Nesting Compositions - After Effects
What it does
Nesting is defined as building a composition that is used in another composition. Precomposing simply means taking one or more layers that are already in a composition and turning them into a composition of their own. Many people can confuse nesting and precomposing, since for both, one composition feeds another composition.
The difference is that, when you nest, you build a composition and set it up to suit your needs, and then place that composition into another composition, along with other nested compositions and layers to create your final product. When you precompose, you take a layer or layers that already live in a composition, and precomposing applies an effect to all of them, to make them move in unison or to apply the same transfer mode to them.
Why it's helpful
Nesting is primarily used for organizational purposes. It treats compositions as a layer in another composition, and it helps to create your overall project. Precomposing makes multiple layers do similar things. It also sets the new composition to the same resolution as the original composition, and sets the new composition’s beginning at the beginning of the original.
Both are good to use when you want to apply complex changes to an entire composition.
How to do it
To precompose one or more layers, select them and hit Shift+Command+C (or right click and select Pre-compose). Notice that the layer is now a composition within the original composition, and a new composition has appeared in your project window. That’s a precomposition.
If you still want to manipulate the original layers, double-click on the new composition in the project window. That will bring you inside the precomposition, and the layers will now be available for manipulation.
To nest, this is simply done by bringing two or more compositions from the Project panel into your Composition panel.